Widely used conventional magnetic recording media such as video tapes, audio tapes, and magnetic disks comprise a nonmagnetic support and provided thereon a magnetic layer comprising particles of ferromagnetic iron oxide, Co-modified ferromagnetic iron oxide, CrO.sub.2, a ferromagnetic alloy, or the like dispersed in a binder.
Along with the desire for higher densities, there is a recent trend toward reducing recording wavelength. As a result, self-demagnetization loss occurring during recording and thickness loss occurring during reproduction, both of which lead to reduced output, have become serious problems when the magnetic layer has a large thickness.
Although reducing the thickness of magnetic layers has been attempted in order to eliminate the above problems, magnetic layer thicknesses of about 2 .mu.m or less are disadvantageous in that the surface of such a thin magnetic layer may be affected by the surface roughness of the nonmagnetic support, resulting in impaired electromagnetic characteristics and reduced D.O. (drop out). This support surface roughness can be avoided by first forming a thick, nonmagnetic undercoating layer on the support surface and then forming an upper magnetic layer, as described in JP-A-57-198536. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) This method, however, has not been satisfactory in that no improvement is attained in head abrasion or durability. This may be attributable to the fact that a conventional nonmagnetic lower layer includes a binder which is a thermosetting resin and hence the magnetic layer is brought into contact with a head or other members without a buffer due to the cured lower nonmagnetic layer, and that the magnetic recording medium having such a lower layer has insufficient flexibility.
Although the above problem may be eliminated by use of a non-curing resin as the binder for the lower layer, the use of a non-curing resin binder causes other problems, for example, surface properties of the magnetic layer are impaired by the non-curing resin resulting in poor electromagnetic characteristics. This is because in the conventional method in which an upper magnetic layer is applied after a lower layer has been applied and dried, the lower layer swells due to the organic solvent contained in the coating fluid for forming the upper layer. This affects the upper layer-forming coating fluid by, for example, causing a turbulent flow therein. The thickness of the magnetic layer may be reduced by reducing the spread rate of the magnetic coating fluid or by lowering the concentration of magnetic coating fluid by adding a large amount of solvent thereto. However, if the spread rate is reduced, the applied coating fluid begins to dry before it has sufficiently leveled, causing coating defects, such as streaks or a stamp pattern in the resulting magnetic layer. Consequently, a considerably poor yield results. Lowering the concentration of the magnetic coating fluid is disadvantageous because, for example, the resulting coating film has a large amount of voids, such that a sufficiently high ferromagnetic particle loading cannot be obtained, and the coating film has insufficient strength. Such defects are serious in the magnetic recording media taught by JP-A-62-154225.
One technique for eliminating these problems is to use a nonmagnetic layer as a lower layer and use a high-concentration magnetic coating fluid applied thinly by means of simultaneous double coating, as described in JP-A-63-191315 and JP-A-63-187418. According to such a technique, yield has been improved significantly and it has become possible to obtain good electromagnetic characteristics. However, a higher density magnetic recording medium is desired.
Attempts have been made to form a magnetic recording medium-including two or more magnetic layers each having heightened output signals at different recording wavelengths. Examples of such recording media include those disclosed in JP-B-37-2218. (The term "JP-B" as used herein means an "examined Japanese patent publication".) These recording media are designed to attain higher output and higher carrier to noise ratio (C/N) by employing an upper magnetic layer and a lower magnetic layer having differing coercive force (H.sub.c), differing maximum magnetic flux density (B.sub.m), and differing ferromagnetic particle size. However, such techniques are not satisfactory because dispersibility of ferromagnetic particles is limited. Also it is difficult to achieve sufficient surface smoothness required by high-density magnetic recording media.